Golf ball

ABSTRACT

A golf ball  2  includes a core  4 , an envelope layer  6  positioned outside the core  4 , a mid layer  8  positioned outside the envelope layer  6 , and a cover  10  positioned outside the mid layer  8 . The core  4  includes a spherical center  12  and a core outer layer  14  positioned outside the center  12 . The center  12  has a diameter of 1 mm or greater and 15 mm or less. The difference (H4−H3) between the JIS-C hardness H4 of the core  4  at its surface and the JIS-C hardness H3 of the core outer layer  14  at its innermost portion is equal to or greater than 10. The cover  10  has a Shore D hardness H7 less than 40. The Shore D hardnesses H5, H6 and H7 of the envelope layer  6 , the mid layer  8 , and the cover  10  satisfy that H5&gt;H6≧H7.

This application is a Continuation of co-pending application Ser. No.12/545,230, filed on Aug. 21, 2009. Priority is claimed to JapanesePatent Application No. 2008-243519 filed on Sep. 24, 2008 and JapanesePatent Application No. 2008-297506 filed on Nov. 21, 2008. The entirecontents of these Japanese Patent Applications are hereby incorporatedby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to golf balls. Specifically, the presentinvention relates to multi-piece golf balls including a center, a coreouter layer, an envelope layer, a mid layer and cover.

2. Description of the Related Art

The greatest interest to golf players concerning golf balls is flightperformance. In particular, golf players place importance on a flightdistance upon a shot with a driver. By using a golf ball that has a longflight distance upon a shot with a driver, golf players can hit a secondshot at a point close to the green. A flight distance upon a shot with adriver correlates with a spin rate. A golf ball with a low spin rate hasexcellent flight performance. In addition, flight performance correlateswith the resilience performance of a golf ball.

Golf players also place importance on spin performance of golf balls. Ifa backspin rate is high, the run is short. By using a golf ball that hasa high backspin rate, golf players can cause the golf ball to stop at atarget point. If a sidespin rate is high, the golf ball tends to curve.By using a golf ball that has a high sidespin rate, golf players canintentionally cause the golf ball to curve. A golf ball with excellentspin performance has excellent controllability. In particular, advancedgolf players place importance on controllability upon a shot with ashort iron.

Golf players also place importance on feel at impact of golf balls. Ingeneral, golf players prefer soft feel at impact.

Golf balls with a cover including a polyurethane are commerciallyavailable. In general, polyurethanes are flexible. Spin is easily givento this golf ball. This cover contributes to the controllability. On theother hand, if this golf ball is hit with a driver, this cover causesexcessive spin. This cover impairs the flight performance.

Golf balls with a core and a thin cover formed from a polyurethane arecommercially available. This core includes a center and a mid layer. Themid layer is formed from a hard synthetic resin. This mid layer achievesan outer-hard/inner-soft structure of the core. When this golf ball ishit with a driver, the core deforms significantly because the cover isthin. The outer-hard/inner-soft structure of the core suppresses spin.This core achieves a long flight distance upon a shot with a driver.When this golf ball is hit with a short iron, a high spin rate isachieved by the flexible cover. This cover contributes to thecontrollability upon a shot with a short iron. This golf ball has hardfeel at impact because the cover is thin and the mid layer is hard.

Various golf balls with a multilayer structure have been proposed. U.S.Pat. No. 6,468,169 (JP-HEI10-328326A) discloses a golf ball including acore, an envelope layer, an inner cover and an outer cover. U.S. Pat.No. 6,271,296 (JP2001-17575A) discloses a golf ball including a core, anenvelope layer, a core outer layer and a cover. JP2002-272880A disclosesa golf ball including a core and a cover. The core consists of a centerand an outer core layer. The cover consists of an inner cover layer andan outer cover layer.

US2003/64828 (JP2003-10359A) discloses a golf ball including a core anda cover. The core consists of a center and a mid layer. US2003/166422(JP2003-205052A) discloses a golf ball including a core and a cover. Thecore consists of a center and a mid layer. US2004/29648 (JP2004-130072A)discloses a golf ball including a core and a cover. The core consists ofa center, a mid layer and an outer layer.

In the golf ball disclosed in U.S. Pat. No. 6,468,169, the cover ishard. The golf ball has inferior controllability. In the golf balldisclosed in U.S. Pat. No. 6,271,296, the envelope layer is hard. Thegolf ball has inferior flight performance upon a shot with a driver. Inthe golf ball disclosed in JP2002-272880A, the cover is hard. The golfball has inferior controllability. In the golf ball disclosed inUS2003/64828, the cover is hard. The golf ball has inferiorcontrollability. In the golf ball disclosed in US2003/166422, the coveris hard. The golf ball has inferior controllability. In the golf balldisclosed in US2004/29648, the cover is hard. The golf ball has inferiorcontrollability.

Golf players' requirements for golf balls have been escalated more thanever. An objective of the present invention is to provide a golf ballthat is excellent in various performance characteristics.

SUMMARY OF THE INVENTION

A golf ball according to the present invention comprises a core, anenvelope layer positioned outside the core, a mid layer positionedoutside the envelope layer, and a cover positioned outside the midlayer. The core includes a center and a core outer layer positionedoutside the center. The center has a diameter of 1 mm or greater and 15mm or less. A difference (H4−H3) between a JIS-C hardness H4 of the coreat its surface and a JIS-C hardness H3 of the core outer layer at itsinnermost portion is equal to or greater than 10. The cover has a ShoreD hardness H7 less than 40. A Shore D hardness H5 of the envelope layer,a Shore D hardness H6 of the mid layer, and a Shore D hardness H7 of thecover satisfy the following mathematical formula (I).

H5>H6≦H7  (I)

In the golf ball according to the present invention, the center, thecore outer layer and the envelope layer achieve an outer-hard/inner-softstructure. In the golf ball, a diameter of the center is small and thehardness difference (H4−H3) of the core outer layer is great. Thus, thestep of hardness at the boundary between the center and the core outerlayer is small. While a conventional golf ball has anouter-hard/inner-soft structure with a hardness distribution havinginferior continuity, the golf ball according to the present inventionhas an outer-hard/inner-soft structure with a hardness distributionhaving excellent continuity. In the golf ball, the spin upon a shot witha driver is suppressed sufficiently. Because the diameter of the centeris small, the center does not impair the resilience performance of thegolf ball. Because the hardness of the cover is low, the covercontributes to the controllability upon a shot with a short iron. Thegolf ball has excellent flight performance and excellentcontrollability.

Preferably, a difference (H4−H1) between the hardness H4 and a JIS-Chardness H1 of the center at its central point is equal to or greaterthan 30. Preferably, a difference (H3−H2) between the hardness H3 and aJIS-C hardness H2 of the center at its surface is equal to or less than35. Preferably, a hardness of the envelope layer is greater than asurface hardness of the core. Preferably, the hardness H6 is greaterthan the hardness H7. Preferably, a difference (H5=H6) between thehardness H5 and the hardness H6 is equal to or greater than 5 and equalto or less than 40. Preferably, a difference (H6−H7) between thehardness H6 and the hardness H7 is equal to or greater than 3 and equalto or less than 28.

Preferably, a JIS-C hardness H1 of the center at its central point isequal to or greater than 20 and equal to or less than 50. Preferably,the hardness H3 is equal to or greater than 45 and equal to or less than75, and the hardness H4 is equal to or greater than 65 and equal to orless than 95. Preferably, the hardness H5 is equal to or greater than50. Preferably, the hardness H5 is equal to or greater than 50 and equalto or less than 80.

Preferably, the hardness H6 is equal to or greater than 30 and equal toor less than 60. Preferably, the hardness H7 is equal to or greater than10 and less than 40.

The center can be formed by crosslinking a rubber composition.Preferably, a principal component of a base material of the rubbercomposition is a polybutadiene. The rubber composition includes sulfuras a crosslinking agent.

Preferably, a principal component of a base material of the envelopelayer is an ionomer resin. Preferably, a principal component of a basematerial of the mid layer is one or more selected from the groupconsisting of polyurethanes, polyesters, polyamides, polyolefins,polystyrenes and ionomer resins. Preferably, a principal component of abase material of the cover is a thermoplastic polyurethane elastomer.

Preferably, the mid layer has a thickness Tm less than 1.2 mm.Preferably, the cover has a thickness Tc of 0.6 mm or less.

Preferably, a sphere consisting of the core and the envelope layer hasan amount of compressive deformation of 2.0 mm or greater and 3.6 mm orless. Preferably, a sphere consisting of the core, the envelope layerand the mid layer has an amount of compressive deformation of 2.3 mm orgreater and 4.0 mm or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a golf ball according toan embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail the present invention based onpreferred embodiments with reference to the accompanying drawing.

Golf ball 2 shown in FIG. 1 includes a core 4, an envelope layer 6positioned outside the core 4, a mid layer 8 positioned outside theenvelope layer 6, and a cover 10 positioned outside the mid layer 8. Thecore 4 includes a spherical center 12 and a core outer layer 14positioned outside the center 12. On the surface of the cover 10, alarge number of dimples 16 are formed. Of the surface of the golf ball2, a part other than the dimples 16 is a land 18. The golf ball 2includes a paint layer and a mark layer on the external side of thecover 10 although these layers are not shown in the drawing.

The golf ball 2 has a diameter of 40 mm or greater and 45 mm or less.From the standpoint of conformity to the rules established by the UnitedStates Golf Association (USGA), the diameter is preferably equal to orgreater than 42.67 mm. In light of suppression of air resistance, thediameter is preferably equal to or less than 44 mm and more preferablyequal to or less than 42.80 mm. The golf ball 2 has a weight of 40 g orgreater and 50 g or less. In light of attainment of great inertia, theweight is preferably equal to or greater than 44 g and more preferablyequal to or greater than 45.00 g. From the standpoint of conformity tothe rules established by the USGA, the weight is preferably equal to orless than 45.93 g.

The center 12 is obtained by crosslinking a rubber composition. Examplesof preferable base rubbers for use in the rubber composition includepolybutadienes, polyisoprenes, styrene-butadiene copolymers,ethylene-propylene-diene copolymers and natural rubbers. In light ofresilience performance, polybutadienes are preferred. When anotherrubber is used in combination with a polybutadiene, it is preferred ifthe polybutadiene is included as a principal component. Specifically,the proportion of the polybutadiene to the entire base rubber ispreferably equal to or greater than 50% by weight and more preferablyequal to or greater than 80% by weight. The proportion of cis-1,4 bondsin the polybutadiene is preferably equal to or greater than 40 mol % andmore preferably equal to or greater than 80 mol %.

The rubber composition of the center 12 includes sulfur. The sulfurserves to crosslink the rubber molecules. The center 12 obtained bysulfur crosslinking is flexible. The center 12 achieves anouter-hard/inner-soft structure of the core 4. The core 4 suppressesspin upon a shot with a driver. The core 4 also contributes to the feelat impact.

In light of resilience performance of the golf ball 2, the amount of thesulfur is preferably equal to or greater than 2.0 parts by weight andparticularly preferably equal to or greater than 3.0 parts by weight,per 100 parts by weight of the base rubber. In light of flexibility ofthe center 12, the amount of the sulfur is preferably equal to or lessthan 10.0 parts by weight and particularly preferably equal to or lessthan 6.5 parts by weight, per 100 parts by weight of the base rubber.

Preferably, the rubber composition of the center 12 includes avulcanization accelerator. The vulcanization accelerator achieves ashort time period for crosslinking the center 12. Guanidine typevulcanization accelerators, thiazole type vulcanization accelerators,sulfenamide type vulcanization accelerators, aldehyde-ammonia typevulcanization accelerators, thiourea type vulcanization accelerators,thiuram type vulcanization accelerators, dithiocarbamate typevulcanization accelerators, xanthate type vulcanization accelerators andthe like can be used. Guanidine type vulcanization accelerators,thiazole type vulcanization accelerators and sulfenamide typevulcanization accelerators are preferred. Two or more types ofvulcanization accelerators may be used in combination.

Examples of guanidine type vulcanization accelerators include1,3-diphenylguanidine, 1,3-di-o-tolylguanidine, 1-o-tolylbiguanide andthe di-o-tolylguanidine salt of dicatechol borate. Specific examples of1,3-diphenylguanidine include trade names “Nocceler D” and “NoccelerD-P” available from Ouchi Shinko Chemical Industrial Co., Ltd., andtrade names “Soxinol D”, “Soxinol DG”, and “Soxinol DO”, available fromSumitomo Chemical Co., Ltd. Specific examples of 1,3-di-o-tolylguanidineinclude trade name “Nocceler DT” available from Ouchi Shinko ChemicalIndustrial Co., Ltd., and trade names “Soxinol DT” and “Soxinol DT-O”available from Sumitomo Chemical Co., Ltd. One specific example of1-o-tolylbiguanide is trade name “Nocceler BG” available from OuchiShinko Chemical Industrial Co., Ltd. One specific example of thedi-o-tolylguanidine salt of dicatechol borate is trade name “NoccelerPR” available from Ouchi Shinko Chemical Industrial Co., Ltd.

Examples of thiazole type vulcanization accelerators include2-mercaptobenzothiazole, di-2-benzothiazolyl disulfide,2-mercaptobenzothiazole zinc salt, 2-mercaptobenzothiazolecyclohexylamine salt, 2-(N,N-diethylthiocarbamoylthio)benzothiazole and2-(4′-morpholinodithio)benzothiazole. Specific examples of2-mercaptobenzothiazole include trade names “Nocceler M” and “NoccelerM-P” available from Ouchi Shinko Chemical Industrial Co., Ltd. Specificexamples of di-2-benzothiazolyl disulfide include trade names “NoccelerDM” and “Nocceler DM-P” available from Ouchi Shinko Chemical IndustrialCo., Ltd. One specific example of 2-mercaptobenzothiazol zinc salt istrade name “Nocceler MZ” available from Ouchi Shinko Chemical IndustrialCo., Ltd. One specific example of 2-mercaptobenzothiazolecyclohexylamine salt is trade name “Nocceler M-60-OT” available fromOuchi Shinko Chemical Industrial Co., Ltd. One specific example of2-(N,N-diethylthiocarbamoylthio)benzothiazole is trade name “Nocceler64” available from Ouchi Shinko Chemical Industrial Co., Ltd. Specificexamples of 2-(4′-morpholinodithio)benzothiazole include trade names“Nocceler MDB” and “Nocceler MDB-P” available from Ouchi Shinko ChemicalIndustrial Co., Ltd.

Examples of sulfenamide type vulcanization accelerators includeN-cyclohexyl-2-benzothiazolyl sulfenamide, N-tert-butyl-2-benzothiazolylsulfenamide, N-oxydiethylene-2-benzothiazolyl sulfenamide andN,N′-dicyclohexyl-2-benzothiazolyl sulfenamide. Specific examples ofN-cyclohexyl-2-benzothiazolyl sulfenamide include trade names “NoccelerCZ” and “Nocceler CZ-G” available from Ouchi Shinko Chemical IndustrialCo., Ltd. Specific examples of N-tert-butyl-2-benzothiazolyl sulfenamideinclude trade names “Nocceler NS” and “Nocceler NS-P” available fromOuchi Shinko Chemical Industrial Co., Ltd. One specific example ofN-oxydiethylene-2-benzothiazolyl sulfenamide is trade name “NoccelerMSA-G” available from Ouchi Shinko Chemical Industrial Co., Ltd.Specific examples of N,N′-dicyclohexyl-2-benzothiazolyl sulfenamideinclude trade names “Nocceler DZ” and “Nocceler DZ-G” available fromOuchi Shinko Chemical Industrial Co., Ltd.

The amount of the vulcanization accelerator is preferably equal to orgreater than 0.5 part by weight and particularly preferably equal to orgreater than 2.0 parts by weight, per 100 parts by weight of the baserubber. The amount of the vulcanization accelerator is preferably equalto or less than 7.0 parts by weight and particularly preferably equal toor less than 5.0 parts by weight, per 100 parts by weight of the baserubber.

In a general golf ball, the rubber composition of the center includes anorganic peroxide. The organic peroxide contributes to the resilienceperformance of the golf ball. On the other hand, the organic peroxideincreases the hardness of the center. The center 12 of the golf ball 2shown in FIG. 1 does not include any organic peroxide. Because of therubber composition, a flexible center 12 is obtained. The rubbercomposition may include an organic peroxide in a small amount.

Preferably, the center 12 includes a reinforcing material. A preferablereinforcing material is silica (white carbon). Silica can achieveappropriate rigidity of the center 12. Dry-process silica andwet-process silica can be used. In light of rigidity of the center 12,the amount of the silica is preferably equal to or greater than 5 partsby weight and particularly preferably equal to or greater than 10 partsby weight, per 100 parts by weight of the base rubber. In light offlexibility of the center 12, the amount of the silica is preferablyequal to or less than 40 parts by weight and particularly preferablyequal to or less than 30 parts by weight, per 100 parts by weight of thebase rubber. A silane coupling agent may be included together withsilica.

For the purpose of adjusting specific gravity and the like, a filler maybe included in the center 12. Examples of suitable fillers include zincoxide, barium sulfate, calcium carbonate and magnesium carbonate. Powderof a metal with a high specific gravity may be included as a filler.Specific examples of metals with a high specific gravity includetungsten and molybdenum. The amount of the filler is determined asappropriate so that the intended specific gravity of the center 12 isaccomplished. A particularly preferable filler is zinc oxide. Zinc oxideserves not only as a specific gravity adjuster but also as acrosslinking activator.

According to need, various additives such as an anti-aging agent, acoloring agent, a plasticizer, a dispersant, a co-crosslinking agent, anorganic sulfur compound and the like are included in the center 12 in anadequate amount. Crosslinked rubber powder or synthetic resin powder maybe also included in the center 12.

In light of durability, the center 12 has at its central point ahardness H1 of preferably 20 or greater, more preferably 25 or greater,and particularly preferably or greater. In light of suppression of spin,the hardness H1 is preferably equal to or less than 50, more preferablyequal to or less than 45, and particularly preferably equal to or lessthan 40. The hardness H1 is measured by pressing a JIS-C type hardnessscale against the central point of a cut plane of the center 12 that hasbeen cut into two halves. For the measurement, an automated rubberhardness measurement machine (trade name “P1”, available from KobunshiKeiki Co., Ltd.), to which this hardness scale is mounted, is used.

The hardness of the center 12 gradually increases from its central pointtoward its surface. The center 12 has a surface hardness H2 greater thanthe central hardness H1. The great surface hardness H2 can achievecontinuity of hardness between the center 12 and the core outer layer14. In this respect, the surface hardness H2 of the center is preferablyequal to or greater than 25, more preferably equal to or greater than30, and particularly preferably equal to or greater than 35. In light offeel at impact, the surface hardness H2 is preferably equal to or lessthan 70, more preferably equal to or less than 60, and particularlypreferably equal to or less than 50. The surface hardness H2 is measuredby pressing a JIS-C type hardness scale against the surface of thecenter 12. For the measurement, an automated rubber hardness measurementmachine (trade name “P1”, available from Kobunshi Keiki Co., Ltd.), towhich this hardness scale is mounted, is used.

In light of feel at impact, the difference (H2−H1) between the hardnessH2 and the hardness H1 is preferably equal to or greater than 1, morepreferably equal to or greater than 3, and particularly preferably equalto or greater than 5. In light of resilience performance, the difference(H2−H1) is preferably equal to or less than 15, more preferably equal toor less than 10, and particularly preferably equal to or less than 7.

In light of feel at impact, the center 12 has an amount of compressivedeformation D1 of preferably 0.5 mm or greater, more preferably 1.0 mmor greater, and particularly preferably 1.1 mm or greater. In light ofresilience performance, the amount of compressive deformation D1 ispreferably equal to or less than 2.5 mm, more preferably equal to orless than 2.3 mm, and particularly preferably equal to or less than 2.0mm.

Upon measurement of the amount of compressive deformation, first, asphere is placed on a hard plate made of metal. Next, a cylinder made ofmetal gradually descends toward the sphere. The sphere, squeezed betweenthe bottom face of the cylinder and the hard plate, becomes deformed. Amigration distance of the cylinder, starting from the state in which aninitial load is applied to the sphere up to the state in which a finalload is applied thereto, is the amount of compressive deformation. Inmeasuring the amount of compressive deformation D1 of the center 12, theinitial load is 0.3 N and the final load is 29.4 N. In measuring: theamount of compressive deformation D2 of the core 4; the amount ofcompressive deformation D3 of a sphere consisting of the core 4 and theenvelope layer 6; the amount of compressive deformation D4 of a sphereconsisting of the core 4, the envelope layer 6 and the mid layer 8; andthe amount of compressive deformation D5 of the golf ball 2, the initialload is 98 N and the final load is 1274 N.

The center 12 has a diameter less than that of the center of a generalgolf ball. Because of the small center 12, the core outer layer 14 canbe formed with a sufficient thickness. The core outer layer 14 canachieve an outer-hard/inner-soft structure with a hardness distributionhaving excellent continuity. The small center 12 suppresses spin. Evenif being flexible, the small center 12 does not impair the resilienceperformance of the golf ball 2. In light of continuity of the hardnessdistribution and resilience performance, the diameter of the center 12is preferably equal to or less than 15 mm, more preferably equal to orless than 12 mm, and particularly preferably equal to or less than 10mm. From the standpoint that the center 12 can contribute to suppressionof spin, the diameter is preferably equal to or greater than 2 mm, morepreferably equal to or greater than 4 mm, and particularly preferablyequal to or greater than 5 mm.

The center 12 has a weight of preferably 0.05 g or greater and 3 g orless. The temperature for crosslinking the center 12 is generally equalto or higher than 140° C. and equal to or lower than 180° C. The timeperiod for crosslinking the center 12 is generally equal to or longerthan 5 minutes and equal to or shorter than 60 minutes. The center 12may be formed with two or more layers. The center 12 may have a rib onthe surface thereof.

The core outer layer 14 is obtained by crosslinking a rubbercomposition. Examples of preferable base rubbers for use in the rubbercomposition include polybutadienes, polyisoprenes, styrene-butadienecopolymers, ethylene-propylene-diene copolymers and natural rubbers. Inlight of resilience performance, polybutadienes are preferred. Whenanother rubber is used in combination with a polybutadiene, it ispreferred if the polybutadiene is included as a principal component.Specifically, the proportion of the polybutadiene to the entire baserubber is preferably equal to or greater than 50% by weight and morepreferably equal to or greater than 80% by weight. The proportion ofcis-1,4 bonds in the polybutadiene is preferably equal to or greaterthan 40 mol % and more preferably equal to or greater than 80 mol %.

In order to crosslink the core outer layer 14, a co-crosslinking agentis preferably used. Examples of preferable co-crosslinking agents inlight of resilience performance include monovalent or bivalent metalsalts of an α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms.Specific examples of preferable co-crosslinking agents include zincacrylate, magnesium acrylate, zinc methacrylate and magnesiummethacrylate. In light of resilience performance, zinc acrylate and zincmethacrylate are particularly preferred.

In light of resilience performance of the golf ball 2, the amount of theco-crosslinking agent is preferably equal to or greater than 10 parts byweight, more preferably equal to or greater than 15 parts by weight, andparticularly preferably equal to or greater than 20 parts by weight, per100 parts by weight of the base rubber. In light of soft feel at impact,the amount of the co-crosslinking agent is preferably equal to or lessthan 50 parts by weight, more preferably equal to or less than 45 partsby weight, and particularly preferably equal to or less than 40 parts byweight, per 100 parts by weight of the base rubber.

Preferably, the rubber composition of the core outer layer 14 includesan organic peroxide together with a co-crosslinking agent. The organicperoxide serves as a crosslinking initiator. The organic peroxidecontributes to the resilience performance of the golf ball 2. Examplesof suitable organic peroxides include dicumyl peroxide,1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane and di-t-butyl peroxide. Inlight of versatility, dicumyl peroxide is preferred.

In light of resilience performance of the golf ball 2, the amount of theorganic peroxide is preferably equal to or greater than 0.1 part byweight, more preferably equal to or greater than 0.3 part by weight, andparticularly preferably equal to or greater than 0.5 part by weight, per100 parts by weight of the base rubber. In light of soft feel at impact,the amount of the organic peroxide is preferably equal to or less than3.0 parts by weight, more preferably equal to or less than 2.5 parts byweight, and particularly preferably equal to or less than 2.0 parts byweight, per 100 parts by weight of the base rubber.

Preferably, the rubber composition of the core outer layer 14 includesan organic sulfur compound. Examples of preferable organic sulfurcompounds include monosubstitutions such as diphenyl disulfide,bis(4-chlorophenyl)disulfide, bis(3-chlorophenyl)disulfide,bis(4-bromophenyl)disulfide, bis(3-bromophenyl)disulfide,bis(4-fluorophenyl)disulfide, bis(4-iodophenyl)disulfide, andbis(4-cyanophenyl)disulfide; disubstitutions such asbis(2,5-dichlorophenyl)disulfide, bis(3,5-dichlorophenyl)disulfide,bis(2,6-dichlorophenyl)disulfide, bis(2,5-dibromophenyl)disulfide,bis(3,5-dibromophenyl)disulfide, bis(2-chloro-5-bromophenyl)disulfideand bis(2-cyano-5-bromophenyl)disulfide; trisubstitutions such asbis(2,4,6-trichlorophenyl)disulfide andbis(2-cyano-4-chloro-6-bromophenyl)disulfide; tetrasubstitutions such asbis(2,3,5,6-tetrachlorophenyl)disulfide; and pentasubstitutions such asbis(2,3,4,5,6-pentachlorophenyl)disulfide andbis(2,3,4,5,6-pentabromophenyl)disulfide. The organic sulfur compoundcontributes to the resilience performance. Particularly preferableorganic sulfur compounds are diphenyl disulfide andbis(pentabromophenyl)disulfide.

In light of resilience performance of the golf ball 2, the amount of theorganic sulfur compound is preferably equal to or greater than 0.1 partby weight and more preferably equal to or greater than 0.2 part byweight, per 100 parts by weight of the base rubber. In light of softfeel at impact, the amount of the organic sulfur compound is preferablyequal to or less than 1.5 parts by weight, more preferably equal to orless than 1.0 part by weight, and particularly preferably equal to orless than 0.8 part by weight, per 100 parts by weight of the baserubber.

For the purpose of adjusting specific gravity and the like, a filler maybe included in the core outer layer 14. Examples of suitable fillersinclude zinc oxide, barium sulfate, calcium carbonate and magnesiumcarbonate.

Powder of a metal with a high specific gravity may be included as afiller. Specific examples of metals with a high specific gravity includetungsten and molybdenum. The amount of the filler is determined asappropriate so that the intended specific gravity of the core outerlayer 14 is accomplished. A particularly preferable filler is zincoxide. Zinc oxide serves not only as a specific gravity adjuster butalso as a crosslinking activator. According to need, various additivessuch as sulfur, an anti-aging agent, a coloring agent, a plasticizer, adispersant and the like are included in the core outer layer 14 in anadequate amount. Crosslinked rubber powder or synthetic resin powder maybe also included in the core outer layer 14.

The hardness of the core outer layer 14 gradually increases from itsinnermost portion to its surface (i.e. the surface of the core 4). Thecore outer layer 14 has a low hardness H3 at its innermost portion and agreat hardness H4 at its surface. The low hardness H3 can achieve thecontinuity of hardness between the center 12 and the core outer layer14. The great hardness H4 achieves the outer-hard/inner-soft structureof the core 4. The core outer layer 14 sufficiently suppresses spin upona shot with a driver.

In light of the resilience performance, the innermost portion hardnessH3 is preferably equal to or greater than 45, more preferably equal toor greater than 55, and particularly preferably equal to or greater than63. In light of continuity of the hardness distribution, the innermostportion hardness H3 is preferably equal to or less than 75, morepreferably equal to or less than 70, and particularly preferably equalto or less than 67. The hardness H3 is measured for a hemisphereobtained by cutting the core 4. The hardness H3 is measured by pressinga JIS-C type hardness scale against the cut plane of the hemisphere. Thehardness scale is pressed against a region surrounded by: a first circlethat is the boundary between the center 12 and the core outer layer 14;and a second circle that is concentric with the first circle and has aradius 1 mm greater than the radius of the first circle. For themeasurement, an automated rubber hardness measurement machine (tradename “P1”, available from Kobunshi Keiki Co., Ltd.), to which thishardness scale is mounted, is used.

In light of achievement of an outer-hard/inner-soft structure, thesurface hardness H4 of the core 4 is preferably equal to or greater than65, more preferably equal to or greater than 75, even more preferablyequal to or greater than 80, and particularly preferably equal to orgreater than 85. In light of feel at impact, the hardness H4 ispreferably equal to or less than 95, more preferably equal to or lessthan 93, even more preferably equal to or less than 92, and particularlypreferably equal to or less than 90. The hardness H4 is measured bypressing a JIS-C type hardness scale against the surface of the core 4.For the measurement, an automated rubber hardness measurement machine(trade name “P1”, available from Kobunshi Keiki Co., Ltd.), to whichthis hardness scale is mounted, is used.

In light of suppression of spin, the difference (H4−H3) between thesurface hardness H4 of the core 4 and the innermost portion hardness H3of the core outer layer 14 is preferably equal to or greater than 10,more preferably equal to or greater than 13, and particularly preferablyequal to or greater than 14. In light of ease of production anddurability, the difference (H4−H3) is preferably equal to or less than25, more preferably equal to or less than 20, and particularlypreferably equal to or less than 18.

In light of achievement of a great difference (H4−H3), the core outerlayer 14 has a thickness of preferably 10 mm or greater, more preferably12 mm or greater, and particularly preferably 14 mm or greater. Thethickness is preferably equal to or less than 20 mm, more preferablyequal to or less than 19 mm, and particularly preferably equal to orless than 18 mm.

During formation of the core outer layer 14, the center 12 is coveredwith two uncrosslinked or semi-crosslinked half shells. These halfshells are compressed and heated. By this heating, a crosslinkingreaction takes place to complete the core outer layer 14. Thecrosslinking temperature is generally equal to or higher than 140° C.and equal to or lower than 180° C. The time period for crosslinking thecore outer layer 14 is generally equal to or longer than 10 minutes andequal to or shorter than 60 minutes.

In light of continuity of the hardness distribution, the difference(H3−H2) between the innermost portion hardness H3 of the core outerlayer 14 and the surface hardness H2 of the center 12 is preferablyequal to or less than 35, more preferably equal to or less than 30, andparticularly preferably equal to or less than 28. The difference (H3−H2)may be zero.

In light of suppression of spin, the difference (H4−H1) between thesurface hardness H4 of the core 4 and the central hardness H1 of thecenter 12 is preferably equal to or greater than 30, more preferablyequal to or greater than 36, and particularly preferably equal to orgreater than 41. In light of ease of production and durability, thedifference (H4−H1) is preferably equal to or less than 65, morepreferably equal to or less than 60, and particularly preferably equalto or less than 51.

In light of feel at impact, the core 4 has an amount of compressivedeformation D2 of preferably 2.3 mm or greater, more preferably 2.6 mmor greater, and particularly preferably 2.8 mm or greater. In light ofresilience performance, the amount of compressive deformation D2 ispreferably equal to or less than 4.0 mm, more preferably equal to orless than 3.6 mm, and particularly preferably equal to or less than 3.2mm.

In light of resilience performance, the core 4 has a diameter ofpreferably 28.0 mm or greater, more preferably 30.0 mm or greater, andparticularly preferably 32.0 mm or greater. In light of durability ofthe golf ball 2, the diameter of the core 4 is preferably equal to orless than 40.2 mm, more preferably equal to or less than 39.9 mm, andparticularly preferably equal to or less than 39.6 mm.

A resin composition is suitably used for the envelope layer 6. Examplesof the base polymer of this resin composition include ionomer resins,polystyrenes, polyolefins, polyurethanes, polyesters and polyamides. Twoor more polymers may be used in combination. Particularly, ionomerresins are preferred. Ionomer resins are highly elastic. As describedlater, the mid layer 8 and the cover 10 of the golf ball 2 are thin.When the golf ball 2 is hit with a driver, the envelope layer 6significantly deforms due to the thinness of the mid layer 8 and thecover 10. The envelope layer 6 including an ionomer resin contributes tothe resilience performance upon a shot with a driver.

Examples of preferable ionomer resins include binary copolymers formedwith an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms. A preferable binary copolymer includes 80% by weight ormore and 90% by weight or less of an α-olefin, and 10% by weight or moreand 20% by weight or less of an α,β-unsaturated carboxylic acid. Thisbinary copolymer provides excellent resilience performance to the golfball 2. Examples of other preferable ionomer resins include ternarycopolymers formed with: an α-olefin; an α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms; and an α,β-unsaturated carboxylate esterhaving 2 to 22 carbon atoms. A preferable ternary copolymer includes 70%by weight or more and 85% by weight or less of an α-olefin, 5% by weightor more and 30% by weight or less of an α,β-unsaturated carboxylic acid,and 1% by weight or more and 25% by weight or less of an α,β-unsaturatedcarboxylate ester. This ternary copolymer provides excellent resilienceperformance to the golf ball 2. For the binary copolymer and ternarycopolymer, preferable α-olefins are ethylene and propylene, whilepreferable α,β-unsaturated carboxylic acids are acrylic acid andmethacrylic acid. A particularly preferable ionomer resin is a copolymerformed with ethylene and acrylic acid or methacrylic acid.

In the binary copolymer and ternary copolymer, some of the carboxylgroups are neutralized with metal ions. Examples of metal ions for usein neutralization include sodium ion, potassium ion, lithium ion, zincion, calcium ion, magnesium ion, aluminum ion and neodymium ion. Theneutralization may be carried out with two or more types of metal ions.Particularly suitable metal ions in light of resilience performance anddurability of the golf ball 2 are sodium ion, zinc ion, lithium ion andmagnesium ion.

Specific examples of ionomer resins include trade names “Himilan 1555”,“Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”, “Himilan1856”, “Himilan 1855”, “Himilan AM7311”, “Himilan AM7315”, “HimilanAM7317”, “Himilan AM7318”, “Himilan MK7320” and “Himilan MK7329”,available from Du Pont-MITSUI POLYCHEMICALS Co., Ltd.; trade names“Surlyn 6120”, “Surlyn 6320”, “Surlyn 6910”, “Surlyn 7930”, “Surlyn7940”, “Surlyn 8140”, “Surlyn 8150”, “Surlyn 8940”, “Surlyn 8945”,“Surlyn 9120”, “Surlyn 9150”, “Surlyn 9910”, “Surlyn 9945”, “SurlynAD8546”, “HPF1000” and “HPF2000”, available from E.I. du Pont de Nemoursand Company; and trade names “IOTEK 7010”, “IOTEK 7030”, “IOTEK 7510”,“IOTEK 7520”, “IOTEK 8000” and “IOTEK 8030”, available from ExxonMobilChemical Corporation. Two or more types of ionomer resins may be used incombination. An ionomer resin neutralized with a monovalent metal ion,and an ionomer resin neutralized with a bivalent metal ion may be usedin combination.

As described later, the envelope layer 6 is hard. Use of an ionomerresin having a high acid content achieves a hard envelope layer 6. Theacid content is preferably equal to or greater than 10% by weight andequal to or less than 30% by weight. Specific examples of ionomer resinshaving a high acid content include the aforementioned “Himilan 1605”,“Himilan 1706”, “Himilan 1707”, “Himilan AM7311”, “Himilan AM7317”,“Himilan AM7318”, “Himilan AM7329”, “Surlyn 6120”, “Surlyn 6910”,“Surlyn 7930”, “Surlyn 7940”, “Surlyn 8140”, “Surlyn 8150”, “Surlyn8940”, “Surlyn 8945”, “Surlyn 9120”, “Surlyn 9150”, “Surlyn 9910”,“Surlyn 9945”, “Surlyn AD8546”, “IOTEK 8000” and “IOTEK 8030”.

An ionomer resin and another resin may be used in combination. In thiscase, in light of resilience performance, the ionomer resin is includedas the principal component of the base polymer. The proportion of theionomer resin to the entire base polymer is preferably equal to orgreater than 50% by weight, more preferably equal to or greater than 70%by weight, and particularly preferably equal to or greater than 85% byweight.

A preferable resin that can be used in combination with an ionomer resinis a styrene block-containing thermoplastic elastomer. This elastomercan contribute to the feel at impact of the golf ball 2. This elastomerdoes not impair the resilience performance of the golf ball 2. Thiselastomer includes a polystyrene block as a hard segment, and a softsegment. A typical soft segment is a diene block. Examples of dienecompounds include butadiene, isoprene, 1,3-pentadiene and2,3-dimethyl-1,3-butadiene. Butadiene and isoprene are preferred. Two ormore compounds may be used in combination.

Examples of styrene block-containing thermoplastic elastomers includestyrene-butadiene-styrene block copolymers (SBS),styrene-isoprene-styrene block copolymers (SIS),styrene-isoprene-butadiene-styrene block copolymers (SIBS), hydrogenatedSBS, hydrogenated SIS and hydrogenated SIBS. Examples of hydrogenatedSBS include styrene-ethylene-butylene-styrene block copolymers (SEBS).Examples of hydrogenated SIS include styrene-ethylene-propylene-styreneblock copolymers (SEPS). Examples of hydrogenated SIBS includestyrene-ethylene-ethylene-propylene-styrene block copolymers (SEEPS).

In light of resilience performance of the golf ball 2, the content ofthe styrene component in the thermoplastic elastomer is preferably equalto or greater than 10% by weight, more preferably equal to or greaterthan 12% by weight, and particularly preferably equal to or greater than15% by weight. In light of feel at impact of the golf ball 2, thecontent is preferably equal to or less than 50% by weight, morepreferably equal to or less than by weight, and particularly preferablyequal to or less than 45% by weight.

In the present invention, styrene block-containing thermoplasticelastomers include alloys of olefin and one or more selected from thegroup consisting of SBS, SIS, SIBS, SEBS, SEPS, SEEPS, and hydrogenatedproducts thereof. An olefin component in the alloy is presumed tocontribute to the improvement of compatibility with ionomer resins. Useof this alloy improves the resilience performance of the golf ball 2. Anolefin having 2 to 10 carbon atoms is preferably used. Examples ofsuitable olefins include ethylene, propylene, butene and pentene.Ethylene and propylene are particularly preferred.

Specific examples of polymer alloys include trade names “RabalonT3221C”, “Rabalon T3339C” “Rabalon SJ4400N”, “Rabalon SJ5400N”, “RabalonSJ6400N”, “Rabalon SJ7400N”, “Rabalon SJ8400N”, “Rabalon SJ9400N” and“Rabalon SR04”, available from Mitsubishi Chemical Corporation. Otherspecific examples of styrene block-containing thermoplastic elastomersinclude a trade name “Epofriend A1010” available from Daicel ChemicalIndustries, Ltd., and a trade name “Septon HG-252” available fromKuraray Co., Ltd.

When an ionomer resin and a styrene block-containing thermoplasticelastomer are used in combination for the envelope layer 6, the weightratio of them is preferably equal to or greater than 50/50 and equal toor less than 97/3. The envelope layer 6 with a weight ratio of 50/50 orgreater contributes to the resilience performance of the golf ball 2. Inthis respect, the ratio is more preferably equal to or greater than70/30 and particularly preferably equal to or greater than 85/15. Theenvelope layer 6 with a weight ratio of 97/3 or less contributes to thefeel at impact of the golf ball 2. In this respect, the ratio is morepreferably equal to or less than 95/10.

The envelope layer 6 is hard. The golf ball 2 with the hard envelopelayer 6 has excellent resilience performance upon a shot with a driver.A sphere consisting of the hard envelope layer 6 and the core 4 canachieve an outer-hard/inner-soft hardness distribution. When the golfball 2 having this hardness distribution is hit with a driver, the spinis suppressed. The synergistic effect of the resilience performance andthe spin suppression achieves excellent flight performance of the golfball 2. The golf ball 2 having this hardness distribution also hasexcellent feel at impact. In light of flight performance and feel atimpact, the envelope layer 6 has a Shore D hardness H5 of preferably 50or greater, more preferably 55 or greater, and particularly preferably57 or greater. In light of feel at impact and durability, the hardnessH5 is preferably equal to or less than 80, more preferably equal to orless than 75, and particularly preferably equal to or less than 72.

In the present invention, the hardness H5 of the envelope layer 6 ismeasured according to the standards of “ASTM-D 2240-68”. For themeasurement, an automated rubber hardness measurement machine (tradename “P1”, available from Kobunshi Keiki Co., Ltd.), to which a Shore Dtype hardness scale is mounted, is used. For the measurement, a sheet,which is formed by hot press and made of the same material as theenvelope layer 6 and which has a thickness of about 2 mm, is used. Priorto the measurement, the sheet is maintained at 23° C. for two weeks. Atthe measurement, three sheets are stacked.

In light of flight performance, the envelope layer 6 has a thickness Tsof preferably 0.5 mm or greater, more preferably 0.7 mm or greater, andparticularly preferably 0.8 mm or greater. In light of feel at impact,the thickness Ts is preferably equal to or less than 2.2 mm, morepreferably equal to or less than 1.5 mm, and particularly preferablyequal to or less than 1.2 mm.

According to need, a coloring agent such as titanium dioxide, a fillersuch as barium sulfate, a dispersant, an antioxidant, an ultravioletabsorber, a light stabilizer, a fluorescent material, a fluorescentbrightener and the like are included in the envelope layer 6 in anadequate amount. For the purpose of adjusting specific gravity, powderof a metal with a high specific gravity such as tungsten, molybdenum andthe like may be included in the envelope layer 6.

For forming the envelope layer 6, known methods such as injectionmolding, compression molding and the like can be used. In light ofproductivity, injection molding is preferred.

In light of feel at impact, the sphere consisting of the core 4 and theenvelope layer 6 has an amount of compressive deformation D3 ofpreferably 2.0 mm or greater, more preferably 2.2 mm or greater, andparticularly preferably 2.4 mm or greater. In light of resilienceperformance, the amount of compressive deformation D3 is preferablyequal to or less than 3.6 mm, more preferably equal to or less than 3.4mm, and particularly preferably equal to or less than 3.2 mm.

The mid layer 8 is formed from a resin composition. Examples of the basepolymer of this resin composition include polyurethanes, polyesters,polyamides, polyolefins, polystyrenes and ionomer resins. Two or morepolymers may be used in combination. Particularly, polyurethanes arepreferred. Polyurethanes are flexible. When the golf ball 2 with the midlayer 8 including a polyurethane is hit with a short iron, the spin rateis high. The mid layer 8 formed from a polyurethane contributes to thecontrollability upon a shot with a short iron.

When the golf ball 2 is hit with a driver, a long iron, or a middleiron, the sphere consisting of the core 4 and the envelope layer 6becomes significantly distorted because the head speed is high. Becausethis sphere has the outer-hard/inner-soft structure as described above,the spin is suppressed. When the golf ball 2 is hit with a short iron,this sphere becomes less distorted because the head speed is low. Whenthe golf ball 2 is hit with a short iron, the behavior of the golf ball2 mainly depends on the mid layer 8 and the cover 10. Because of the midlayer 8 including the polyurethane, a high spin rate is obtained whenthe golf ball 2 is hit with a short iron, even though the above spherehas the outer-hard/inner-soft structure. This high spin rate achievesexcellent controllability.

A polyurethane and another resin may be used in combination for the midlayer 8. In this case, in light of spin performance and feel at impact,the polyurethane is included as the principal component of the basepolymer. The proportion of the polyurethane to the entire base polymeris preferably equal to or greater than 50% by weight, more preferablyequal to or greater than 70% by weight, and particularly preferablyequal to or greater than 85% by weight.

For the mid layer 8, thermoplastic polyurethanes and thermosettingpolyurethanes can be used. In light of productivity, thermoplasticpolyurethanes are preferred. A thermoplastic polyurethane includes apolyurethane component as a hard segment, and a polyester component or apolyether component as a soft segment. Examples of the curing agent forthe polyurethane component include alicyclic diisocyanates, aromaticdiisocyanates and aliphatic diisocyanates.

Examples of alicyclic diisocyanates include 4,4′-dicyclohexylmethanediisocyanate (H₁₂MDI), 1,3-bis(isocyanatemethyl)cyclohexane (H₆XDI),isophorone diisocyanate (IPDI) and trans-1,4-cyclohexane diisocyanate(CHDI). In light of versatility and processability, H₁₂MDI is preferred.

Examples of aromatic diisocyanates include 4,4′-diphenylmethanediisocyanate (MDI) and toluene diisocyanate (TDI). One example ofaliphatic diisocyanates is hexamethylene diisocyanate (HDI).

Specific examples of thermoplastic polyurethanes include trade names“Elastollan XNY80A”, “Elastollan XNY85A”, “Elastollan XNY90A”,“Elastollan XNY97A”, “Elastollan XNY585” and “Elastollan XKP016N”,available from BASF Japan Ltd.; and trade names “RESAMINE P4585LS” and“RESAMINE PS62490”, available from Dainichiseika Color & Chemicals Mfg.Co., Ltd.

The mid layer 8 may be formed from a composition including athermoplastic polyurethane and a isocyanate compound. During or afterforming the mid layer 8, the polyurethane is crosslinked with theisocyanate compound.

The mid layer 8 has a Shore D hardness H6 of 60 or less. Use of theflexible mid layer 8 can achieve excellent controllability upon a shotwith a short iron. In light of controllability, the hardness H6 is morepreferably equal to or less than 57, even more preferably equal to orless than 54, and particularly preferably equal to or less than 47. Ifthe hardness H6 is excessively low, the flight performance upon a shotwith a driver is insufficient. In this respect, the hardness H6 ispreferably equal to or greater than 30, more preferably equal to orgreater than 35, and particularly preferably equal to or greater than38. The hardness H6 of the mid layer 8 is measured by the same method asthat for the hardness H5 of the envelope layer 6.

The mid layer 8 has a thickness Tm of 1.2 mm or less. As describedabove, the mid layer 8 is flexible. The mid layer 8 is disadvantageousto the resilience coefficient of the golf ball 2. Upon a shot with adriver, the core 4 and the envelope layer 6 also deform significantly.By setting the thickness Tm to be equal to or less than 1.2 mm, the midlayer 8 does not have a significantly adverse effect on the resiliencecoefficient upon a shot with a driver, even if the mid layer 8 isflexible. The mid layer 8 with a thickness Tm of 1.2 mm or less does notimpair the flight performance of the golf ball 2. In light of flightperformance, the thickness Tm is more preferably equal to or less than1.0 mm and particularly preferably equal to or less than 0.8 mm. Inlight of controllability upon a shot with a short iron, the thickness Tmis preferably equal to or greater than 0.1 mm, more preferably equal toor greater than 0.2 mm, and particularly preferably equal to or greaterthan 0.3 mm.

According to need, a coloring agent such as titanium dioxide, a fillersuch as barium sulfate, a dispersant, an antioxidant, an ultravioletabsorber, a light stabilizer, a fluorescent material, a fluorescentbrightener and the like are included in the mid layer 8 in an adequateamount.

For forming the mid layer 8, known methods such as injection molding,compression molding, cast molding and the like can be used. The midlayer 8 may be formed by applying the solution or dispersion liquid ofthe resin composition to the surface of the envelope layer 6.

In light of feel at impact, the sphere consisting of the core 4, theenvelope layer 6 and the mid layer 8 has an amount of compressivedeformation D4 of preferably 2.3 mm or greater, more preferably 2.4 mmor greater, and particularly preferably 2.5 mm or greater. In light ofresilience performance, the amount of compressive deformation D4 ispreferably equal to or less than 4.0 mm, more preferably equal to orless than 3.9 mm, and particularly preferably equal to or less than 3.8mm.

The cover 10 is formed from a resin composition. Examples of the basepolymer of this resin composition include polyurethanes, polyesters,polyamides, polyolefins, polystyrenes and ionomer resins. Two or morepolymers may be used in combination. Particularly, polyurethanes arepreferred. Polyurethanes are flexible. When the golf ball 2 with thecover 10 including a polyurethane is hit with a short iron, the spinrate is high. The cover 10 formed from a polyurethane contributes to thecontrollability upon a shot with a short iron. The polyurethane alsocontributes to the scuff resistance of the cover 10.

When the golf ball 2 is hit with a driver, a long iron, or a middleiron, the sphere consisting of the core 4 and the envelope layer 6becomes significantly distorted because the head speed is high. Becausethis sphere has an outer-hard/inner-soft structure as described above,the spin is suppressed. When the golf ball 2 is hit with a short iron,this sphere becomes less distorted because the head speed is low. Whenthe golf ball 2 is hit with a short iron, the behavior of the golf ball2 mainly depends on the mid layer 8 and the cover 10. Because of thecover 10 including the polyurethane, a high spin rate is obtained whenthe golf ball 2 is hit with a short iron, even though the above spherehas the outer-hard/inner-soft structure. This high spin rate achievesexcellent controllability.

A polyurethane and another resin may be used in combination for thecover 10. In this case, in light of spin performance and feel at impact,the polyurethane is included as the principal component of the basepolymer. The proportion of the polyurethane to the entire base polymeris preferably equal to or greater than 50% by weight, more preferablyequal to or greater than 70% by weight, and particularly preferablyequal to or greater than 85% by weight.

For the cover 10, thermoplastic polyurethanes and thermosettingpolyurethanes can be used. In light of productivity, thermoplasticpolyurethanes are preferred. The thermoplastic polyurethanes describedabove for the mid layer 8 can be used for the cover 10. Polyurethaneseach including a polyurethane component for which an alicyclicdiisocyanate is used as the curing agent are particularly preferred. Analicyclic diisocyanate does not have any double bond in the main chain.In the cover 10 formed from a polyurethane for which an alicyclicdiisocyanate is used as the curing agent, yellowing of the cover 10 issuppressed. In addition, because the polyurethane for which an alicyclicdiisocyanate is used as the curing agent has excellent strength, thecover 10 is prevented from being damaged.

The cover 10 may be formed from a composition including a thermoplasticpolyurethane and an isocyanate compound. During or after forming thecover 10, the polyurethane is crosslinked with the isocyanate compound.

The cover 10 has a Shore D hardness H7 less than 40. Use of the flexiblecover 10 can achieve excellent controllability upon a shot with a shortiron. In light of controllability, the hardness H7 is more preferablyequal to or less than 38 and particularly preferably equal to or lessthan 36. If the hardness H7 is excessively low, the flight performanceupon a shot with a driver is insufficient. In this respect, the hardnessH7 is preferably equal to or greater than 10, more preferably equal toor greater than 15, and particularly preferably equal to or greater than20. The hardness H7 of the cover 10 is measured by the same method asthat for the hardness H5 of the envelope layer 6.

The cover 10 has a thickness Tc of 0.6 mm or less. As described above,the cover 10 is flexible. The cover 10 is disadvantageous to theresilience coefficient of the golf ball 2. Upon a shot with a driver,the core 4 and the envelope layer 6 also deform significantly. Bysetting the thickness Tc to be equal to or less than 0.6 mm, the cover10 does not have a significantly adverse effect on the resiliencecoefficient upon a shot with a driver, even if the cover 10 is flexible.The cover 10 with a thickness Tc of 0.6 mm or less does not impair theflight performance of the golf ball 2. In light of flight performance,the thickness Tc is more preferably equal to or less than 0.5 mm andparticularly preferably equal to or less than 0.4 mm. In light ofcontrollability upon a shot with a short iron, the thickness Tc ispreferably equal to or greater than 0.1 mm, more preferably equal to orgreater than 0.2 mm, and particularly preferably equal to or greaterthan 0.3 mm.

According to need, a coloring agent such as titanium dioxide, a fillersuch as barium sulfate, a dispersant, an antioxidant, an ultravioletabsorber, a light stabilizer, a fluorescent material, a fluorescentbrightener and the like are included in the cover 10 in an adequateamount.

For forming the cover 10, known methods such as injection molding,compression molding, cast molding and the like can be used. When formingthe cover 10, the dimples 16 are formed by pimples formed on the cavityface of a mold. The cover 10 may be formed by applying the solution ordispersion liquid of the resin composition to the surface of the midlayer 8. A sphere with the cover 10 having a smooth surface from theabove application may be formed, and is placed into a mold to form thedimples 16 thereon.

The golf ball 2 satisfies the following mathematical formula.

H5>H6≧H7  (I)

-   -   H5: Shore D hardness of the envelope layer 6    -   H6: Shore D hardness of the mid layer 8    -   H7: Shore D hardness of the cover 10

In the golf ball 2, the hardness H5 of the envelope layer 6 is great.The envelope layer 6 achieves an outer-hard/inner-soft structure. Theenvelope layer 6 suppresses the spin when the golf ball 2 is hit with adriver. The envelope layer 6 contributes to the flight performance upona shot with a driver. In the golf ball 2, the hardness H6 of the midlayer 8 and the hardness H7 of the cover 10 are low. When the golf ball2 with the mid layer 8 and the cover 10 is hit with a short iron, a highspin rate is obtained. The cover 10 contributes to the controllabilityupon a shot with a short iron.

Preferably, the golf ball 2 satisfies the following mathematical formula(II).

H5>H6>H7  (II)

In other words, in the golf ball 2, the hardness H6 of the mid layer 8is greater than the hardness H7 of the cover 10. As described above, thebase polymer of the mid layer 8 is a polyurethane, and the base polymerof the cover 10 is also a polyurethane. By using different types ofpolyurethanes for the mid layer 8 and the cover 10, the golf ball 2 inwhich the hardness H6 is greater than the hardness H7 is obtained. Ifthe mid layer 8 is not provided, the hardness distribution has a greatstep at the boundary between the envelope layer 6 and the cover 10. Inthe golf ball 2 with the mid layer 8 in which the hardness H6 is lessthan the hardness H5 of the envelope layer 6 and greater than thehardness H7 of the cover 10, the hardness distribution does not havesuch a great step. The mid layer 8 contributes to the feel at impact.The golf ball 2 has excellent flight performance, excellentcontrollability and excellent feel at impact.

In light of feel at impact, the golf ball 2 has an amount of compressivedeformation D5 of preferably 2.0 mm or greater, more preferably 2.2 mmor greater, and particularly preferably 2.4 mm or greater. In light ofresilience performance, the amount of compressive deformation D5 ispreferably equal to or less than 3.3 mm, more preferably equal to orless than 3.1 mm, and particularly preferably equal to or less than 2.9mm.

In light of flight performance, controllability and feel at impact, thedifference (H5−H6) between the hardness H5 and the hardness H6 ispreferably equal to or greater than 5 and equal to or less than 40, morepreferably equal to or greater than 10 and equal to or less than 35, andparticularly preferably equal to or greater than 15 and equal to or lessthan 30.

In light of flight performance, controllability and feel at impact, thedifference (H6−H7) between the hardness H6 and the hardness H7 ispreferably equal to or greater than 3 and equal to or less than 28, morepreferably equal to or greater than 6 and equal to or less than 25, andparticularly preferably equal to or greater than 9 and equal to or lessthan 21.

In light of flight performance upon a shot with a driver, the sum(Tm+Tc) of the thickness Tm of the mid layer 8 and the thickness Tc ofthe cover 10 is preferably equal to or less than 1.6 mm, more preferablyequal to or less than 1.5 mm, and particularly preferably equal to orless than 1.4 mm. In light of ease of producing the mid layer 8 and thecover 10, the sum (Tm+Tc) is preferably equal to or greater than 0.2 mmand more preferably equal to or greater than 0.3 mm.

The golf ball 2 may include a reinforcing layer between the envelopelayer 6 and the mid layer 8. The reinforcing layer firmly adheres to theenvelope layer 6 and also to the mid layer 8. The reinforcing layerprevents separation of the mid layer 8 from the envelope layer 6.

As the base polymer of the reinforcing layer, a two-component curingtype thermosetting resin is suitably used. Examples of two-componentcuring type thermosetting resins include epoxy resins, urethane resins,acrylic resins, polyester resins and cellulose resins. In light ofstrength and durability of the reinforcing layer, two-component curingtype epoxy resins and two-component curing type urethane resins arepreferred.

The reinforcing layer may include additives such as a coloring agent(typically, titanium dioxide), a phosphate-based stabilizer, anantioxidant, a light stabilizer, a fluorescent brightener, anultraviolet absorber, an anti-blocking agent and the like. The additivesmay be added to the base material of the two-component curing typethermosetting resin, or may be added to the curing agent of thetwo-component curing type thermosetting resin.

The reinforcing layer is obtained by applying, to the surface of theenvelope layer 6, a liquid that is prepared by dissolving or dispersingthe base material and the curing agent in a solvent. In light ofworkability, application with a spray gun is preferred. After theapplication, the solvent is volatilized to permit a reaction of the basematerial with the curing agent, thereby forming the reinforcing layer.

In light of adhesion, the reinforcing layer has a thickness ofpreferably 3 μm or greater and more preferably 5 μm or greater. In lightof ease of forming the reinforcing layer, the thickness is preferablyequal to or less than 300 μm, more preferably equal to or less than 50μm, and particularly preferably equal to or less than 20 μm. Thethickness is measured by observing a cross section of the golf ball 2with a microscope. When the envelope layer 6 has concavities andconvexities on its surface from surface roughening, the thickness of thereinforcing layer is measured at a convex part.

In light of adhesion, the reinforcing layer has a pencil hardness ofpreferably 4B or harder and more preferably B or harder. In light ofreduced loss of the power transmission from the mid layer 8 to theenvelope layer 6 upon a hit of the golf ball 2, the reinforcing layerhas a pencil hardness of preferably 3H or softer. The pencil hardness ismeasured according to the standards of “JIS K5400”.

EXAMPLES Example 1

A rubber composition (1) was obtained by kneading 100 parts by weight ofa high-cis polybutadiene (trade name “BR-730”, available from JSRCorporation), 5 parts by weight of zinc oxide, an appropriate amount ofbarium sulfate, 10 parts by weight of silica (trade name “Nipsil AQ”,available from Tosoh Silica Corporation), 3.4 parts by weight of sulfur,2.20 parts by weight of a vulcanization accelerator (the aforementioned“Nocceler CZ”), and 2.26 parts by weight of another vulcanizationaccelerator (the aforementioned “Soxinol DG”). This rubber composition(1) was placed into a mold including upper and lower mold halves eachhaving a hemispherical cavity, and heated at 150° C. for 5 minutes toobtain a center with a diameter of 5.0 mm.

A rubber composition (4) was obtained by kneading 100 parts by weight ofa high-cis polybutadiene (the aforementioned “BR-730”), 37 parts byweight of zinc diacrylate, 5 parts by weight of zinc oxide, anappropriate amount of barium sulfate, 0.5 part by weight of diphenyldisulfide, and 0.8 part by weight of dicumyl peroxide (available fromNOF Corporation). Two half shells were formed from this rubbercomposition (4). The center was covered with the two half shells. Thecenter and the half shells were placed into a mold including upper andlower mold halves each having a hemispherical cavity, and heated at 170°C. for 20 minutes to obtain a core with a diameter of 39.5 mm. Theamount of barium sulfate was adjusted such that the specific gravity ofa mid layer agreed with the specific gravity of the center and theweight of a golf ball was 45.4 g.

A resin composition (d) was obtained by kneading 50 parts by weight ofan ionomer resin (the aforementioned “Himilan 1605”), 50 parts by weightof another ionomer resin (the aforementioned “Himilan AM7329”), and 14parts by weight of titanium dioxide with a twin-screw kneading extruder.The core was placed into a mold including upper and lower mold halveseach having a hemispherical cavity. The core was covered with the resincomposition (d) by injection molding to form an envelope layer with athickness of 1.0 mm.

A paint composition (trade name “POLIN 750LE”, available from SHINTOPAINT CO., LTD.) including a two-component curing type epoxy resin as abase polymer was prepared. The base material liquid of this paintcomposition includes 30 parts by weight of a bisphenol A type solidepoxy resin and 70 parts by weight of a solvent. The curing agent liquidof this paint composition includes 40 parts by weight of a modifiedpolyamide amine, 55 parts by weight of a solvent, and 5 parts by weightof titanium oxide. The weight ratio of the base material liquid to thecuring agent liquid is 1/1. This paint composition was applied to thesurface of the envelope layer with a spray gun, and maintained at 23° C.for 24 hours to obtain a reinforcing layer with a thickness of 10 μm.

A resin composition (b) was obtained by kneading 100 parts by weight ofa thermoplastic polyurethane elastomer (the aforementioned “ElastollanXNY90A”) and 4 parts by weight of titanium dioxide with a twin-screwkneading extruder. Two half shells were obtained from this resincomposition (b) by compression molding. The sphere consisting of thecore, the envelope layer and the reinforcing layer was covered withthese two half shells. The half shells and the sphere were placed into amold including upper and lower mold halves each having a hemisphericalcavity, and compression molding was performed to form a mid layer with athickness of 0.3 mm.

A resin composition (c) was obtained by kneading 100 parts by weight ofa thermoplastic polyurethane elastomer (the aforementioned “ElastollanXNY80A”) and 4 parts by weight of titanium dioxide with a twin-screwkneading extruder. Two half shells were obtained from this resincomposition (c) by compression molding. The sphere consisting of thecore, the envelope layer, the reinforcing layer and the mid layer wascovered with these two half shells. The half shells and the sphere wereplaced into a final mold that includes upper and lower mold halves eachhaving a hemispherical cavity and that has a large number of pimples onits cavity face, and compression molding was performed to form a coverwith a thickness of 0.3 mm. A large number of dimples having a shapethat is the inverted shape of the pimples were formed on the cover. Aclear paint including a two-component curing type polyurethane as a basematerial was applied to this cover to obtain a golf ball of Example 1with a diameter of 42.7 mm and a weight of about 45.4 g.

Examples 2 to 5 and Comparative Examples 1 to 6

Golf balls of Examples 2 to 5 and Comparative Examples 1 to 6 wereobtained in a similar manner as Example 1, except the specifications ofthe center, the core outer layer, the envelope layer, the mid layer andthe cover were as shown in the following Tables 3 to 5. The rubbercomposition of the core is shown in detail in the following Table 1. Theresin compositions of the envelope layer, the mid layer and the coverare shown in detail in the following Table 2. The golf ball ofComparative Example 1 does not have a core outer layer.

[Shot with Driver (W#1)]

A driver with a titanium head (trade name “SRIXON W505”, available fromSRI Sports Limited, shaft hardness: X, loft angle: 8.5°) was attached toa swing machine available from Golf Laboratories, Inc. A golf ball washit under the condition of a head speed of 50 m/sec, and the distancefrom the launch point to the stop point was measured. In addition, thebackspin rate was measured immediately after the hit. The average valueof data obtained by 12 measurements is shown in the following Tables 3to 5.

[Shot with Sand Wedge (SW)]

A sand wedge was attached to a swing machine available from True TemperCo. A golf ball was hit under the condition of a head speed of 21 m/sec,and the backspin rate was measured. The average value of data obtainedby 12 measurements is shown in the following Tables 3 to 5.

[Feel at Impact]

Ten golf players hit golf balls with drivers, and were asked about feelat impact. The evaluation was categorized as follows based on the numberof golf players who answered, “the impact shock was small and the feelat impact was excellent”.

A: 8 or more

B: 6 to 7

C: 4 to 5

D: 3 or less

The results are shown in the following Tables 3 to 5.

TABLE 1 Composition of Core (parts by weight) (1) (2) (3) (4)Polybutadiene 100 100 100 100 Zinc diacrylate — — 15 37 Zinc oxide 5 5 55 Barium sulfate Appropriate Appropriate Appropriate Appropriate amountamount amount amount Silica 10 30 — — Diphenyl — — 0.5 0.5 disulfideDicumyl — — 0.7 0.7 peroxide Sulfur 3.4 3.4 — — Accelerator CZ 2.20 2.20— — Accelerator DG 2.26 2.26 — —

TABLE 2 Compositions of Envelope Layer, Mid Layer and Cover (parts byweight) (a) (b) (c) (d) (e) Elastollan XNY97A 100 — — — — ElastollanXNY90A — 100  — — — Elastollan XNY80A — — 100  — — Himilan 1605 — — — 50— Himilan AM7329 — — — 50 — Himilan 1555 — — — — 46 Himilan 1557 — — — —46 Rabalon T3221C — — — — 8 Titanium dioxide 4  4  4 14 4 Hardness(Shore-D) 47 38 26 67 57

TABLE 3 Results of Evaluation Example 1 Example 2 Example 3 Example 4Center Composition (1) (2) (1) (2) Diameter (mm) 5 10 5 10 Centralhardness H1 (JIS C) 30 40 30 40 Surface hardness H2 (JIS C) 35 47 35 47Crosslinking temperature (° C.) 150 150 150 150 Crosslinking time (min)5 5 5 5 Core Composition (4) (4) (4) (4) outer Thickness (mm) 17.2514.75 17.25 14.75 layer Innermost hardness H3 (JIS C) 63 67 63 67Crosslinking temperature (° C.) 170 170 170 170 Crosslinking time (min)20 20 20 20 Core Surface hardness H4 (JIS C) 81 81 81 81 Difference (H3− H2) 28 20 28 20 Difference (H4 − H1) 51 41 51 41 Difference (H4 − H3)18 14 18 14 Compressive deformation D2 (mm) 3.0 3.0 3.0 3.0 EnvelopeComposition (d) (d) (d) (e) layer Thickness Ts (mm) 1.0 0.8 0.8 1.2Hardness H5 (Shore D) 67 67 67 57 Mid Composition (b) (a) (a) (a) layerThickness Tm (mm) 0.3 0.3 0.5 0.2 Hardness H6 (Shore D) 38 47 47 47Cover Composition (c) (b) (c) (c) Thickness Tc (mm) 0.3 0.5 0.3 0.2Hardness H7 (Shore D) 26 38 26 26 Ball Compressive deformation D5 (mm)2.6 2.7 2.7 2.8 W#1 Spin (rpm) 2100 2100 2150 2400 Flight distance (m)278.5 278.0 277.5 274.0 SW Spin (rpm) 7000 6900 6950 7150 Feel at impactA A A A

TABLE 4 Results of Evaluation Compa. Compa. Compa. Example 5 Example 1Example 2 Example 3 Center Composition (1) (2) (4) (1) Diameter (mm) 520 39.5 5 Central hardness H1 (JIS C) 30 40 62 30 Surface hardness H2(JIS C) 35 51 81 35 Crosslinking temperature (° C.) 150 150 170 150Crosslinking time (min) 5 5 20 5 Core Composition (4) (4) — (4) outerThickness (mm) 17.25 9.75 — 17.25 layer Innermost hardness H3 (JIS C) 6372 — 63 Crosslinking temperature (° C.) 170 170 — 170 Crosslinking time(min) 20 20 — 20 Core Surface hardness H4 (JIS C) 81 81 81 (H2) 81Difference (H3 − H2) 28 21 — 28 Difference (H4 − H1) 51 41 19 51Difference (H4 − H3) 18 9 — 18 Compressive deformation D2 (mm) 3.0 2.952.9 3.0 Envelope Composition (d) (d) (d) (d) layer Thickness Ts (mm) 1.00.8 0.9 0.8 Hardness H5 (Shore D) 67 67 67 67 Mid Composition (b) (b)(a) (e) layer Thickness Tm (mm) 0.3 0.2 0.3 0.6 Hardness H6 (Shore D) 3838 47 57 Cover Composition (b) (c) (c) (a) Thickness Tc (mm) 0.3 0.6 0.40.2 Hardness H7 (Shore D) 38 26 38 47 Ball Compressive deformation D5(mm) 2.6 2.6 2.6 2.5 W#1 Spin (rpm) 2050 2450 2450 2050 Flight distance(m) 279.0 273.5 273.5 279.5 SW Spin (rpm) 6900 7250 7200 6650 Feel atimpact A A A C

TABLE 5 Results of Evaluation Compa. Compa. Compa. Example 4 Example 5Example 6 Center Composition (2) (1) (2) Diameter (mm) 10 5 23 Centralhardness H1 (JIS C) 40 30 41 Surface hardness H2 (JIS C) 47 35 52Crosslinking temperature (° C.) 150 150 150 Crosslinking time (min) 5 55 Core Composition (4) (4) (4) outer Thickness (mm) 14.75 17.25 8.25layer Innermost hardness H3 (JIS C) 67 63 73 Crosslinking temperature (°C.) 170 170 170 Crosslinking time (min) 20 20 20 Core Surface hardnessH4 (JIS C) 81 81 81 Difference (H3 − H2) 20 28 21 Difference (H4 − H1)41 51 40 Difference (H4 − H3) 14 18 8 Compressive deformation D2 (mm)3.0 3.0 2.95 Envelope Composition (e) (d) (d) layer Thickness Ts (mm)0.8 1.2 0.8 Hardness H5 (Shore D) 57 67 67 Mid Composition (d) — (b)layer Thickness Tm (mm) 0.6 — 0.2 Hardness H6 (Shore D) 67 — 38 CoverComposition (c) (b) (c) Thickness Tc (mm) 0.2 0.4 0.6 Hardness H7 (ShoreD) 26 38 26 Ball Compressive deformation D5 (mm) 2.5 2.5 2.6 W#1 Spin(rpm) 2200 2000 2500 Flight distance (m) 276.0 279.5 273.0 SW Spin (rpm)6800 6700 7300 Feel at impact C C A

As shown in Tables 3 to 5, the golf balls of Examples are excellent forall the evaluation items. From the results of evaluation, advantages ofthe present invention are clear.

The golf ball according to the present invention can be used for playinggolf on a golf course and practicing at a driving range. The abovedescription is merely for illustrative examples, and variousmodifications can be made without departing from the principles of thepresent invention.

1. A golf ball comprising a core, an envelope layer positioned outsidethe core, a mid layer positioned outside the envelope layer, and a coverpositioned outside the mid layer, wherein: the core includes a centerand a core outer layer positioned outside the center; a difference(H4−H3) between a JIS-C hardness H4 of the core at its surface and aJIS-C hardness H3 of the core outer layer at its innermost portion isequal to or greater than
 10. 2. The golf ball according to claim 1,wherein the center has a diameter of 1 mm or greater than 15 mm or less.3. The golf ball according to claim 1, wherein the envelope layer has athickness of 0.5 mm or greater and 2.2 mm or less.
 4. The golf ballaccording to claim 1, wherein the mid layer has a thickness of 0.1 mm orgreater than 1.2 mm or less.
 5. The golf ball according to claim 1,wherein a Shore D hardness H6 of the mid layer and a Shore D hardness H7of the cover satisfy the following mathematical formula H6>H7.
 6. Thegolf ball according to claim 5, wherein the hardness H7 is equal to orless than
 40. 7. The golf ball according to claim 5, wherein a Shore Dhardness H5 of the envelope layer, the hardness H6 and the hardness H7satisfy the following mathematical formula H5>H6≧H7.
 8. The golf ballaccording to claim 1, wherein a difference (H2−H1) between a JIS-Chardness H1 of the center at its central point and a JIS-C hardness H2of the center at its surface is equal to or less than
 15. 9. The golfball according to claim 1, wherein the envelope layer has a firstthickness, the mid layer has a second thickness, and the cover has athird thickness, the second thickness is no greater than the firstthickness, and the second thickness is no greater than the thirdthickness.
 10. A golf ball comprising a core, an envelope layerpositioned outside the core, a mid layer positioned outside the envelopelayer, and a cover positioned outside the mid layer, wherein: the coreincludes a center and a core outer layer positioned outside the center;the core has a diameter of 28.0 mm or greater and 40.2 mm or less;difference (H3−H2) between a JIS-C hardness H3 of the core outer layerat its innermost portion and a JIS-C hardness H2 of the center at itssurface is equal to or less than 35; a Shore D hardness H6 of the midlayer and a Shore D hardness H7 of the cover satisfy the followingmathematical formulaH6>H7.
 11. The golf ball according to claim 10, wherein a difference(H2−H1) between the hardness H2 and a JIS-C hardness H1 of the center atits central point is equal to or less than
 15. 12. A golf ballcomprising a core, a cover positioned outside the core, and at least onelayer positioned between the cover and the core wherein: the coreincludes a center and a core outer layer positioned outside the center;the center has a diameter of equal to or less than 15 mm; a difference(H4−H3) between a JIS-C hardness H4 of the core at its surface and aJIS-C hardness H3 of the core outer layer at its innermost portion isequal to or greater than
 10. 13. A golf ball comprising a core, a midlayer positioned outside the core, a cover positioned outside the midlayer and at least one layer positioned between the cover and the corewherein: the core includes a center and a core outer layer positionedoutside the center; a difference (H3−H2) between a JIS-C hardness H3 ofthe core outer layer at its innermost portion and a JIS-C hardness H2 ofthe center at its surface is equal to or less than 35; a Shore Dhardness H6 of the mid core layer and a Shore D hardness H7 of the coversatisfy the following mathematical formulaH6>H7.